1. Field of the Invention
The present invention relates to a boot for constant-velocity universal joint. More particularly, it relates to a boot for constant-velocity universal joint, boot which covers constant-velocity universal joint, an indispensable component part for the joint of drive shaft for front-wheel-drive vehicle, to inhibit water and dust from coming in the joint element of constant-velocity universal joint.
2. Description of the Related Art
Constant-velocity universal joints have been used for driving-power transmission systems for transmitting driving power from automotive engines to drive wheels, in particular, for drive shafts for front-wheel-drive vehicles. The constant-velocity universal joints have been provided with boots installed for sealing greases for lubricating the joint elements therein, and for inhibiting water and dust from coming in the joint elements.
For example, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-3,950 discloses a conventional boot for such constant-velocity universal joints, one of known conventional boots therefor. As illustrated in FIG. 5, the conventional boot comprises a major-diameter cylindrical portion 81, a minor-diameter cylindrical portion 82, and a bellows 83. The major-diameter cylindrical portion 81 is fastened to a not-shown joint outer race. The minor-diameter cylindrical portion 82 has a smaller inside and outside diameters than those of the major-diameter cylindrical portion 81, and is fastened to a shaft 91 by a fastener fitting 90, such as clamp or band. The bellows 83 connects the major-diameter cylindrical portion 81 with the minor-diameter cylindrical portion 82 integrally, and is formed as a truncated cone shape substantially.
When the conventional boot for constant-velocity universal joint is put in service, it rotates together with the joint outer race, to which the major-diameter cylindrical portion 81 is fastened, and together with the shaft 91, to which the minor-diameter cylindrical portion 82 is fastened. Then, as shown in FIG. 5, when the shaft 91 inclines nearer with respect to the joint outer race to change an angle that the joint outer race and the shaft 91 make (hereinafter referred to as “joint angle”), the bellows 83, which rotates together with the joint outer race and shaft 91, deforms in compliance with the changing joint angle, thereby sealing the joint element of constant-velocity universal joint.
Note that a general conventional boot for constant-velocity universal joint comprises a rising wall 85, which extends from one of the opposite ends of the minor-diameter cylindrical portion 82 that faces the bellows 83 and to the top of the first crest 84 of the bellows 83. Moreover, the rising wall 85 exhibits a rising angle “θ,” which has been usually set to fall in a range of from 65 to 85 degrees approximately, to an imaginary central line of the general conventional boot or that of the shaft 91.
However, when the constant-velocity universal joint rotates at a large joint angle, the conventional constant-velocity universal-joint boot, which has been deformed to bend or curve greatly, rotates together with the constant-velocity universal joint. Accordingly, the rising wall 85 has been deformed greatly. Consequently, the conventional boot might have been associated with the following problems.
For example, when the shaft 91 inclines more with respect to the joint outer race, a curvature point upon curving or bending the conventional constant-velocity universal-joint boot deviates more from the center in the axial direction of the boot and comes nearer to the side of the major-diameter cylindrical portion 81, which is disposed closer to the joint element between the joint outer race and the shaft 91. Moreover, when the conventional boot is curved or bent, compression stress acts onto the conventional boot on a joint-angle narrowing-down side (i.e., the right-hand side of FIG. 5, that is, the shaft 91 inclines nearer with respect to the joint outer race); and tensile stress acts onto it on the opposite joint-angle widening-up side (i.e., the left-hand side of FIG. 5).
As a result, on the compression side in which the joint angle narrows down, the crests of the bellows 83, which are disposed nearer to the major-diameter cylindrical portion 81, are compressed so that the roots, which are disposed nearer to the major-diameter cylindrical portion 81, are pulled in diametrically inward. Therefore, in the bellows 83, which is disposed nearer to the minor-diameter cylindrical portion 82, a compression-side section 84a of the first crest 84, which neighbors on the minor-diameter cylindrical portion 82, has come to fall down toward the major-diameter cylindrical portion 81, even though the compression-side section 84a is present on the compression side in which the joint angle narrows down.
On the other hand, on the tensile side in which the joint angle widens up, large tensile force acts onto the bellows 83, which is disposed nearer to the major-diameter cylindrical portion 81. However, since the facing compression-side section 84a falls down toward the major-diameter cylindrical portion 81 as described above, an opposite tensile-side section 84b of the first crest 84, which neighbors on the minor-diameter cylindrical portion 82, has come to fall down toward the minor-diameter cylindrical portion 82, even though the tensile-side section 84b is present on the tensile side in which the joint angle widens up.
When the thus bent or curved conventional constant-velocity universal-joint boot rotates, the first crest 84 of the bellows 83, which neighbors on the minor-diameter cylindrical portion 82, is put in such a state that the compression-side section 84a falls down toward the major-diameter cylindrical portion 81, and in such a state that the tensile-side section 84b falls down toward the minor-diameter cylindrical portion 82, alternately and repetitively. Accordingly, the rising wall 85 has come to be put in such a state that it falls down to exhibit a diminishing rising angle when it coincides with a compression-side section 85a, and in such a state that it gets up to exhibit an enlarging rising angle when it coincides with a tensile-side section 85b, alternately and repetitively. Consequently, as illustrated in FIG. 6, the fastener fitting 90 has come to be put in such a state that it sinks downward at a compression-side section 82a, and in such a state that it floats upward at a tensile-side section 82b, alternately and repetitively, in one of the opposite ends of the minor-diameter cylindrical portion 82 that neighbors on the bellows 83.
Under the circumstances, the larger the joint angle, which the joint outer race and shaft 19 make, becomes, the greater the first crest 84 of the bellows 83, which neighbors on the minor-diameter cylindrical portion 82, inclines. Accordingly, the rising wall 85 of the bellows 83 deforms more greatly so that the fastener fitting 90 sinks downward and floats upward more greatly. Consequently, the one of the opposite ends of the minor-diameter cylindrical portion 82, which is disposed on the side of the bellows 83, is subjected to a pumping action that pulls in grease, which is held inside the conventional boot for constant-velocity universal joint, toward the minor-diameter cylindrical portion 82 and then discharges the grease to the outside of the conventional boot. As a result, there might be such a fear that grease leakage occurs.
In the meantime, Japanese Unexamined Utility Model Publication (KOKAI) Gazette No. 6-73,533 discloses a boot for constant-velocity universal joint, boot which is provided with a retaining cylindrical portion. The retaining cylindrical portion is formed by enlarging the axial length of the minor-diameter cylindrical portion, and is thereby disposed nearer to a part of the minor-diameter cylindrical portion, which adjoins the bellows, than the fastener fitting, which is placed adjacent to the opening end of the minor-diameter cylindrical portion, is disposed. In this conventional boot, it is believed that it might be possible to solve the aforementioned grease-leakage problem because the retaining cylindrical portion, which is disposed nearer to the bellows than the fastener fitting is disposed, exists in the minor-diameter cylindrical portion.
However, the conventional boot for constant-velocity universal joint that is disclosed in Japanese Unexamined Utility Model Publication (KOKAI) Gazette No. 6-73,533 has upsized, because it comes to have an axial length that is prolonged by an axial length of the retaining cylindrical portion. On the other hand, in order to inhibit the conventional boot from upsizing, the bellows should have an axial length that is shortened by an axial length of the retaining cylindrical portion. As a result, the resulting shortened bellows might exhibit degraded durability, or it might have become difficult to give the resultant shortened bellows necessary functions securely.